Most people have a natural lordotic curve in the lumbar region of their back and so arching, even just a little, is natural and should be encouraged. Forcing oneself to lie perfectly flat on the bench can in fact emphasise a rounded back, the same rounded back we so often preach about avoiding during deadlifts and squats. For bodybuilders, emphasis on the pecs can still be maintained despite this often thought of as a powerlifting-style technique.